Copernican Revolution

Ptolemy lived roughly five centuries after Aristotle in the Greek colony in Egypt

-Although he believed in the Aristotelian universe, he was interested in a different problem—the motion of the planets. -He was a brilliant mathematician, and he used his talents to create a mathematical description of the motions he saw in the heavens. -For him, first principles took second place to mathematical precision.

Although Mars moves steadily along its orbit, as seen from Earth, it appears to slow to a stop and move westward (retrograde) as Earth passes it.

-As the planetary orbits do not lie in precisely the same plane, a planet does not resume its eastward motion in precisely the same path it followed earlier. -Consequently, it describes a loop whose shape depends on the angle between the orbital planes.

Imagine that you are in a race car, driving rapidly along the inside lane of a circular racetrack.

-As you pass slower cars driving in the outer lanes, they fall behind. -If you did not know you were moving, it would seem that the cars in the outer lanes occasionally slowed to a stop and then backed up for a short interval. •The same thing happens as Earth passes a planet such as Mars.

The second theme is the quest to understand planetary motion.

-Astronomers built elaborate mathematical models, but they could not predict precisely the motion of the visible planets along the ecliptic. -That mystery was finally solved when Isaac Newton described gravity and orbital motion in the late 1600s.

The most astonishing consequence of the Copernican hypothesis was not what it said about the sun but what it said about Earth.

-By placing the sun at the center, Copernicus made Earth move along an orbit like the other planets. -By making Earth a planet, Copernicus revolutionized humanity's view of its place in the universe and triggered a controversy that would eventually bring the astronomer Galileo Galilei before the Inquisition—a controversy over the nature of scientific truth that continues even today.

To the ancients, the universe was small.

-Earth lay at the center surrounded by crystalline shells carrying the planets, and the starry sphere lay just beyond the outermost shell. •To understand their astronomy, you must put aside your 21st century knowledge and think as they did two millennia ago.

He distributed this commentary in handwritten form, without a title, and in some cases anonymously—to friends and astronomical correspondents.

-He may have been cautious out of modesty, out of respect for the Church, or out of fear that his revolutionary ideas would be attacked unfairly. -After all, the place of the Earth was a controversial theological subject

You would not have expected Nicolaus Copernicus to trigger a revolution in astronomy and science

-He was born in 1473 to a merchant family in Poland. -Orphaned at the age of 10, he was raised by his uncle, an important bishop, who sent him to the University of Cracow and then to the best universities in Italy. -He studied law and medicine and pursued a lifelong career as an important administrator in the Church. -Nevertheless, he had a passion for astronomy.

Although Copernicus proposed a revolutionary idea in making the planetary system heliocentric, he was a classical astronomer—with tremendous respect for the old concept of uniform circular motion.

-In fact, Copernicus objected strongly to Ptolemy's use of the equant. -It seemed arbitrary to Copernicus—a direct violation of the elegance of Aristotle's philosophy of the heavens. -He called equants 'monstrous' in that they violated both geocentrism and uniform circular motion

Columbus had to convince the Queen that the world was quite small—so small he could sail to the orient by heading west.

-In making his sales pitch, he underestimated the size of Earth and overestimated the eastward extent of Asia. -So, he thought China and Japan were within a few days' sailing distance of Spain. -If North America had not been in his way, he and his crew would have starved to death long before they reached Japan.

The Copernican model is inaccurate.

-It includes uniform circular motion. -Thus, it does not precisely describe the motions of the planets.

•As you study the history of astronomy, you will notice that two themes twist through the story. •One theme is the struggle to understand the place of Earth.

-It seemed obvious to the ancients that Earth was the center of everything but, as we now know, that is not true. -The debate over the place of Earth involved deep theological questions and eventually led Galileo before the Inquisition

•Scholars and educated people knew Aristotle's astronomy well. •You may have heard the common misconception that Christopher Columbus had to convince Queen Isabella of Spain that the world was round and not flat.

-Like all educated people of her time, the Queen knew the world was round. -Aristotle said so

• One reason he hesitated was that the idea of a heliocentric universe was highly controversial. •This was a time of rebellion in the Church.

-Martin Luther was speaking harshly about fundamental Church teachings. -Others—both scholars and scoundrels—were questioning the authority of the Church. -Even matters as abstract as astronomy could stir controversy.

A first principle is something that is obviously true.

-Once a principle is recognized as true, whatever can be logically derived from it must also be true. -However, what was obviously true to the ancients is not so obvious to us today.

For most scholars, questioning these principles was not an option.

-Over the course of centuries, Aristotle's proposed geometry of the heavens had become linked with the teachings of the Church. -According to the Aristotelian universe, the most perfect region was in the heavens and the most imperfect at Earth's center. -This classical geocentric universe matched the commonly held Christian geometry of heaven and hell, and anyone who criticized the Ptolemaic model was questioning Aristotle's geometry and indirectly challenging belief in heaven and hell.

Plato and Aristotle, the two greatest philosophers of ancient Greece, both influenced the history of astronomy.

-Plato (427?-347 BC) wrote about moral responsibility, ethics, the nature of reality, and the ideals of civil government. -His student Aristotle (384-322 BC) wrote on almost every area of knowledge and is probably the most famous philosopher in history. -These two philosophers established the first widely accepted ideas about the structure of the universe

His close connection with the Church notwithstanding, Copernicus began to consider an alternative to the Ptolemaic universe, probably while he was still at university.

-Sometime before 1514, he wrote an essay that discussed his hypothesis that the sun, not Earth, was the center of the universe. -That is, he proposed that the universe was heliocentric—or sun-centered. -To explain the daily and annual cycles of the sky, he proposed that Earth rotated on its axis and revolved around the sun.

•Only a few centuries ago, astronomers were struggling to understand the sky. •In the process, they invented a new way of understanding nature—a new way of knowing about the physical world.

-That new way of knowing is based on the comparison of theories and evidence. -Today, that new way of knowing is called science.

However, De Revolutionibus failed in one critical way.

-The Copernican model could not predict the positions of the planets any more accurately than the Ptolemaic system could. •To understand why it failed this critical test, you must understand Copernicus and his world.

As Copernicus imposed uniform circular motion on his model, it could not accurately predict the motions of the planets.

-The Prutenic Tables (1551) were based on the Copernican model, and they were not significantly more accurate than the 13th century Alfonsine Tables that were based on Ptolemy's model. -Both could be in error by as much as 2°, which is four times the angular diameter of the full moon.

Although astronomers throughout Europe read and admired De Revolutionibus, they did not usually accept the Copernican hypothesis.

-The mathematics was elegant and the astronomical observations and calculations were of tremendous value. -Nevertheless, few astronomers believed, at first, that the sun actually was the center of the planetary system and that Earth moved.

However, the Copernican hypothesis that the universe is heliocentric was correct—given how little astronomers of the time knew of other stars and galaxies.

-The planets circle the sun, not Earth. -So, the universe that Copernicus knew was heliocentric.

•Aristotle's universe—as embodied in Ptolemy's mathematical model—dominated ancient astronomy. •However, it was wrong.

-The universe is not geocentric. -The planets don't follow circles at uniform speeds

Plato argued that the perfect geometrical figure was a sphere.

-Then, the heavens—which everyone agreed were perfect—must be made up of spheres. -The natural motion of a sphere is rotation, and the only perfect motion is uniform motion. -So, the heavenly spheres were thought to move in uniform circular motion.

When Plato, Aristotle, and Ptolemy said Earth was the center of the universe, they were thinking of a very small universe.

-They didn't know about stars and galaxies. -Earlier, you distinguished among the solar system, which contains the sun and planets; a galaxy, which contains billions of stars; and the universe, which contains many billions of galaxies

Copernicus studied the Ptolemaic universe and probably found it difficult, at first, to consider alternatives.

-Throughout his life, he was associated with the Church—which had adopted many of Aristotle's ideas. -His uncle was an important bishop in Poland and, through his uncle's influence, Copernicus was appointed a canon at the cathedral in Frauenberg—at the unusually young age of 24. -This gave Copernicus an income, although he remained at the universities in Italy

Why did classical astronomers conclude the heavens were made up of spheres?

-Today, scientific arguments depend on evidence and theory. -In classical times, they started from first principles.

In devising his model, Copernicus returned to a strong belief in uniform circular motion.

Although he did not need epicycles to explain retrograde motion, he discovered that the sun, moon, and planets suffered other smaller variations in their motions that he could not explain with uniform circular motion centered on the sun

In the middle of the 13th century, a team of astronomers supported by King Alfonso X of Castile studied the Almagest for 10 years.

Although they did not revise the theory very much, they simplified the calculation of the positions of the planets using the Ptolemaic system and published the result as The Alfonsine Tables—the last great attempt to make the Ptolemaic system of practical use

When he left the universities, he joined his uncle and served as his secretary and personal physician until his uncle died in 1512.

At that point, Copernicus moved into quarters adjoining the cathedral in Frauenburg, where he served as canon for the rest of his life.

•Copernicus worked on his book De Revolutionibus Orbium Coelestium (The Revolutions of the Celestial Spheres) over a period of many years and was essentially finished by about 1529. •Yet, he hesitated to publish it, even though other astronomers knew of his theories.

Church officials concerned about the reform of the calendar sought his advice and looked forward to the publication of his book

In the Copernican system, Earth moves faster along its orbit than the planets that lie farther from the sun.

Consequently, Earth periodically overtakes and passes these planets.

•Although his essay discusses every major aspect of his later work, it did not include observations and calculations to add support. •His ideas needed further work.

He began gathering observations and making detailed calculations in order to publish a book that would demonstrate the truth of his revolutionary ideas

Another reason Copernicus may have hesitated was that his work was incomplete.

His model could not accurately predict planetary positions, and he continued to refine it.

He even allowed the speed of the planets to vary slightly as they circled Earth.

In these ways, he weakened the principles of geocentrism and uniform circular motion.

•At first, the Ptolemaic system predicted the positions of the planets well. •As centuries passed, though, errors accumulated.

Islamic and later European astronomers tried to update the system, computing new constants and adjusting epicycles

How the Copernican hypothesis was gradually recognized as correct has been called the Copernican revolution

It was not just the adoption of a new idea but a total change in the way astronomers think about the place of the Earth.

Three important ideas show how first principles influenced early descriptions of the universe and its motions.

One, ancient philosophers and astronomers accepted as first principles that Earth was located at the center of the universe and that the heavens moved in uniform circular motion.

The most important factor, though, may be the elegance of the idea.

Placing the sun at the center of the universe produced a symmetry among the motions of the planets that is pleasing to the eye as well as to the intellect.

•Today, astronomers recognize that those variations are typical of objects following elliptical orbits. •Copernicus, though, held firmly with uniform circular motion.

So, he had to introduce small epicycles to reproduce these minor variations in the motions of the sun, moon, and planets

Also, Earth's place in astronomical theory was linked to the geometry of heaven and hell

So, moving Earth from its central place was a controversial and perhaps heretical idea.

Science and its methods of investigation did not exist in ancient Greece.

So, when Plato and Aristotle turned their minds to the problem of the structure of the universe, they made use of a process common to their time—reasoning from first principles.

The most important idea in the book was the placing the sun at the center of the universe.

That single innovation had an astonishing consequence—the retrograde motion of the planets was immediately explained in a straightforward way without the large epicycles that Ptolemy had used.

•Copernicus could explain retrograde motion without epicycles. •That was impressive.

The Copernican system was elegant and simple compared with the whirling epicycles and off-center equants of the Ptolemaic system.

If you had sat beside Copernicus in his astronomy classes, you would have studied the Ptolemaic universe.

The central location of Earth was widely accepted, and everyone knew that the heavens moved by the combination of uniform circular motions.

Why that hypothesis gradually won acceptance—in spite of the inaccuracy of the epicycles and deferents—is a question historians still debate.

There are probably a number of reasons—including the revolutionary temper of the times.

In the Copernican model, all the planets were treated the same.

They all followed orbits that circled the sun at the center

The great philosophers of ancient Greece wrote about many different subjects, including what they saw in the sky.

Those writings became the foundation upon which later astronomers built modern astronomy

Two, the evidence—the observed motion of the planets—did not fit the theory very well.

The retrograde loops the planets made were very difficult to explain using geocentrism and uniform circular motion.

In the Ptolemaic model, Mercury and Venus were treated differently from the rest of the planets.

Their epicycles had to remain centered on the Earth-sun line.

Third, Claudius Ptolemy attempted to explain the motion of the planets mathematically—by

devising a small circle rotating along the edge of a larger circle that enclosed a slightly off-center Earth.

It took hard work and years of effort, but

the passions of astronomy gripped some of the greatest minds in history and drove them to try to understand the sky.

Thus, classical philosophers argued that the daily motion of the heavens around Earth and the motions of the seven planets

the sun and moon were counted as planets—against the background of the stars had to be produced by the combination of uniformly rotating spheres carrying objects around in perfect circles.

Finally, in 1540, he allowed the visiting astronomer Joachim Rheticus (1514-1576) to publish an account of the Copernican universe in Rheticus's book Prima Narratio (First Narrative).

•In 1542, Copernicus sent the manuscript for De Revolutionibus off to be printed. •He died in the spring of 1543—before the printing was completed.